The present invention relates to a disk brake cylinder with a parking brake mechanism and to a disk brake comprising such a cylinder.
It is known that disk brakes, which are mounted on the rear wheels of vehicles, act as service brakes, which is their main function, and as parking brakes.
For this purpose, the cylinders of the brakes comprise an interior parking brake mechanism which is operated by pulling on a cable.
One known example of such a mechanism is the one in which the cable ends at a lever which turns a shaft. This shaft is secured to one of two circular plates which face one another and trap between them metal balls which are housed in grooves to form a ball ramp. When the plate secured to the shaft, known as the input plate, turns with respect to the second plate, which can move in terms of translation and is commonly known as the moving plate, the balls run along their grooves, first of all at the bottoms of these grooves then rising up onto the escapement zones bordering said grooves. By running first of all along the bottoms of the grooves, the balls begin by taking up the play in the parking brake mechanism. By then running along the escapement zones, the ball ramps move the two plates apart.
Through a stack of components, this relative movement of the two plates causes the brake shoes to clamp around the disk.
This parking brake mechanism is satisfactory as far as its operation is concerned,
However, the inventors underlying the present invention have noticed that during the phase in which the balls are running along the bottoms of the grooves, the translational displacement of the moving plate is often not great enough to completely take up the play in the parking brake mechanism.
In consequence, this play does not become fully taken up until the balls are already running along their escapement zones.
When the play has been fully compensated for, there then remains only a small translational displacement of the moving plate left for actually applying the shoes to the disk.
This problem is all the more troublesome when the user of the vehicle tends on his own account to limit the force he applies to the parking brake actuating lever when he has moved it through a certain travel.
Thus, if, without knowing it, the user devotes most of the travel of the actuating lever to taking up the mechanical play in the parking brake mechanism, he will not provide sufficient force to completely finish applying the parking brake.
The present invention aims to provide a simple yet economical solution to the abovementioned problem.
A subject of the present invention is a disk brake cylinder comprising a piston capable of moving axially toward a disk and containing a parking brake mechanism consisting of:
an input shaft bearing against an axial stop of the cylinder preventing said shaft from moving in the opposite direction to the disk, said shaft being capable of being turned when the parking brake is operated; and
a ball-ramp mechanism, comprising at least a first groove borne by the input shaft, at least a second groove facing the first groove, borne by a member capable of pushing the piston toward the disk and at least one ball trapped between these grooves, each groove comprising an escapement zone along which, when the input shaft turns with respect to said member, the ball rolls and, initially, takes up the play in the mechanism and then moves said member axially toward the piston with respect to the input shaft,
characterized in that the escapement zone of the groove has an exclusively convex profile.
The disk brake cylinder according to the invention is advantageous in that the convex shape of the escapement zone of the groove allows the ball very quickly to take up the play that there is in the parking brake mechanism.
Thus, the force provided by a user on a parking brake actuating lever can be devoted almost in its entirety to applying the parking brake, without increasing the travel of the brake actuating lever. Furthermore, the convex profile of the escapement zone of the groove is recommended because the force to be provided by the user to take up the play is relatively small by comparison with the force needed to apply the brake. In consequence, when taking up the play, it is not disadvantageous for a significant axial displacement of the moving plate to be brought about for a small rotation of the input plate.
The convex profile of the escapement zone may vary. For example, it may be an arc of a circle extended by a straight line or it may be a portion of an ellipse, or alternatively it may a logarithmic shape, polynomial shape or a combination of conical sections.
Another subject of the invention is a disk brake, characterized in that it comprises a cylinder as described hereinabove.
With a view to making the invention easy to understand, one embodiment thereof, given by way of nonlimiting example, will now be described with reference to the appended drawing(.)